ABSORPTION COEFFICIENTS

For an instrument to sound good you must hear a balance of its high and low frequencies. If your room is too dead in the high frequencies the instrument sounds lifeless and tubby, whereas if it's too live in the high frequencies the instrument sounds distant and has no presence. To balance a room you must have a true balance of the high frequencies and the low frequencies with the right amount of deadness to liveness.

The coefficient of absorption is the basis of acoustic treatment and is the amount of sound energy a surface absorbs and reflects and is measured at different frequencies. If we say that a surface material has an absorption coefficient of 0.25 we are saying that the surface will absorb 25% of the incident acoustic energy, while reflecting back 75% of the total acoustic energy at the specified frequency. It can be displayed as a chart like this:

Absorption Coefficient Chart for typical surfaces

 

If you look at the brick figures you will see that a brick wall will reflect almost all the incident sound energy whilst a wall covered with 25mm(1") Rockwool will absorb fro 80 - 90% of the high frequencies but only 35 - 60% of the low frequencies. Whilst most people think that heavy curtains are a good deadener according to the chart they work only reasonably (45%) at high frequencies and not much at all at low frequencies.

This is the major problem experienced by studios who treat their rooms by putting drapes, carpet or egg cartons on the walls. Basically they are lowering the reflections (reverberation time) of the room BUT ONLY MARGINALLY IN THE HIGH FREQUENCY RANGE! The lows are still humming around the room and the resultant reverberation field creates what we call a muddy - woofy sound. The solution is always to add highs to the recorded signal to return the top end yet really what needs to happen is the low end (100 - 1000Hz) reverberation time needs to be lowered in the recording room and probably in the control room for you to even hear the difference.

Alternatively, look at the absorption figures for a product called Insulco Semi Rigid, 50mm (2") thick. This is a product made from compressed fibreglass and only takes up 50mm (2") of wall space.

Insulco Semi Rigid 50mm

As you can see this stuff really works. It actually absorbs the same amount at 125Hz that curtains absorb at high frequencies. (These figures actually state that it absorbs over 100% of the sound, I think the scale was made before products like Insulco were invented!!) I'm sure overseas visitors to this site can find your local equivalent if you ask around.

When purchasing insulation ask for the absorption coefficient figures

To see a list of typical coefficient figures click here

These absorption coefficients are measured with the material mounted flat onto a surface. Another aspect to consider is how to mount the material on the wall. The following diagram shows what happens if you lift the acoustic material off the wall.

Insulation relative to the wall

As you can see there is an added advantage if you actually lift the insulation off the wall because the point of maximum energy in a wave is at the highest point in the wave which is at the 1/4 wavelength point. So if you, for example, create a box frame 100mm(4") deep and mount the 50mm (2") Insulation flush with the front of the box it will be 50mm(2") off the wall. This will lower the effective frequency of maximum absorption.